The present invention relates to a multilateral wellbore system.
The multilateral wellbore system of the present invention comprises a main wellbore extending into an earth formation, a branch wellbore extending from a selected location of the main wellbore into the earth formation and a casing arranged in the main wellbore, such wellbore system being generally referred to as multilateral system. The branch wellbore can be created together with the main wellbore in a single drilling procedure, or can be created at a later stage after the main wellbore has been in operation for a period of time.
In case the branch wellbore is created at such later stage it is generally undesirable that drilling fluid and/or drill cuttings enter the interior of the casing of the main wellbore. Furthermore it is generally undesirable that hydrocarbon fluid flows from the earth formation into the casing at the junction of the main wellbore and the branch wellbore.
It is an object of the invention to provide an adequate multilateral wellbore system which prevents undesirable inflow of drilling fluid into the casing during drilling of the branch wellbore, and which furthermore prevents undesirable inflow of hydrocarbon fluid into the casing at the junction of the main wellbore and the branch wellbore.
In accordance with the invention there is provided a wellbore system comprising a main wellbore extending into an earth formation, a branch wellbore extending from a selected location of the main wellbore into the earth formation, a casing arranged in the main wellbore, a branching device arranged in the casing and connected to a conduit extending through the casing to a wellbore facility at surface, the branching device having a main bore in fluid communication with the wellbore facility via the conduit, and a branch bore providing fluid communication between the main bore and the branch wellbore via a window opening provided in the casing, wherein a seal is provided between said body and the inner surface of the casing so as to prevent fluid communication between the window opening and the interior of the casing.
The window opening is in fluid communication with the branch bore of the branching device and with the branch wellbore. As the seal prevents fluid communication between the window opening and the interior of the casing, drilling fluid present in the branch bore and the branch wellbore during drilling of the latter is prevented from entering the interior of the casing. The seal also prevents any hydrocarbon fluid present in the branch bore and the branch wellbore during hydrocarbon fluid production from entering the interior of the casing.
Suitable the main wellbore is an existing wellbore and the branch wellbore is drilled a period of time after the main wellbore has become operational to produce hydrocarbon fluid.
The main wellbore generally extends from surface through an overburden layer and a cap rock layer into a hydrocarbon fluid reservoir of the earth formation. The branch wellbore can suitably be drilled into a hydrocarbon fluid containing zone of the earth formation at a relatively large distance from the main wellbore if the branching device is located relatively high in the main wellbore, for example in the overburden layer.
Suitably the main wellbore is an existing wellbore and the branch wellbore is drilled a period of time after the main wellbore has become operational to produce hydrocarbon fluid.